Diesel engines, because of the way they operate, emit soot particles or very fine droplets of condensate or a conglomerate of the two (particulates) as well as typical harmful gasoline engine exhausts (i.e., HC and CO). These “particulates” (herein Diesel soot), are rich in condensed, polynuclear hydrocarbons, some of which may be carcinogenic.
As the awareness of the danger Diesel soot presents to health collides with the need for greater fuel efficiency that Diesel engines provide, regulations have been enacted curbing the amount of Diesel soot permitted to be emitted. To meet these challenges, soot filters have been used. The filters have had many configurations as exemplified by GB 1,014,498 and U.S. Pat. No. 4,828,807. The most common and useful filters have tended to be porous ceramic honeycombs that have plugged channels such that the exhaust gas must enter a channel and pass through the wall of the channel as exemplified by U.S. Pat. No. 4,329,162.
Early attempts to make more thermal shock resistant honeycombs, for example, for use in heat regenerators and catalytic converters focused on the use of ceramics having a low coefficient of thermal expansion, such as described in U.S. Pat. Nos. 4,304,585 and 4,598,054. Nevertheless the art is replete with methods of improving the shock resistance of ceramic honeycombs, for example by assembling smaller honeycombs with layers between the smaller honeycombs (i.e., segmented honeycombs). The layers are well known and have included all manner of ceramic cements, foamed or unfoamed, with differing additives such as ceramic or metal fibers, organic materials such as pore formers and binders. Exemplary patents include those previously mentioned and, for example, U.S. Pat. Nos. 3,251,403; 4,335,783; 4,381,815; 4,810,554; 4,953,627; 5,914,187 and 7,087,286. Unfortunately, all of these result in significantly greater forming complexity and increase in pressure drop, for example, when the honeycomb is used as a filter with inlet and outlet channels.
Another method for increasing the thermal shock resistance of ceramic honeycombs include, for example, creating slits radially or axially in the honeycombs to make the honeycombs more compliant due to hoop and axial stresses, such as in U.S. Pat. Nos. 3,887,741 and 3,983,283. This method to increase the thermal shock resistance, unfortunately, tends to result in fragile honeycombs resulting in more handling damage during the manufacture and to complexity in forming the radial grooves.
What is needed is a Diesel particulate filter that avoids one or more problems of the prior art such as one of the aforementioned problems. In particular, it would be desirable to provide a Diesel particulate filter that maximizes the effective filtration area while smoothing out temperature differences within the catalyst due to combustion of differing species along the length of the filter (i.e., more thermal shock resistant). It would also be desirable when doing so to minimize the pressure drop increase associated with other methods used to improve thermal shock of the honeycombs.